In the manufacture of lightweight paper grades, such as newsprint and fine paper, the web is commonly dried on a series of steam-heated drying cylinders. The wet web is pressed directly onto the cylinders by a series of tensioned, permeable fabrics or felts.
In a conventional double-felted, two tier dryer group, the wet web passes from one cylinder to the next in a generally serpentine fashion through long, unsupported "open draws". The majority of the water vapor that leaves the sheet or web is released in these open draws.
Problems have been experienced during operation of conventional dryer groups with regard to sheet flutter during movement of the web through a open draws. This problem particularly occurs in the "wet end" of the dryer, where the web is still quite wet.
A fluttering web is subject to frequent web breaks, which are expensive and time-consuming to correct. Even an occasional web break is a very big problem. It can damage the felt and even the machine, and it inevitably causes production of paper to stop until any necessary repairs can be made and the web can be re-threaded in the running machine.
While a web break is being corrected, a web of undried paper as wide as the machine (often about 30 feet or nine meters wide) and miles (several km.) long is formed and must be collected, broken up, mixed with a much larger quantity of water, and recycled in the paper machine.
Machine speeds, and thus the amount of paper a machine could produce, were limited prior to the present invention by the need to avoid an excessive number of web breaks by keeping the web speed low enough to minimize its flutter in open draws. Even after taking this precaution, web breaks were a common occurrence.
Also, in conventional dryer groups, problems are caused by cross-directional sheet shrinkage and inefficient ventilation of evaporated water.
Additionally, conventional double felted two tier dryer groups typically require threading ropes in order to thread a tail of the web.
Some of the problems with sheet flutter, sheet shrinkage, and vapor ventilation have been solved by using one or more top-felted single tier dryer groups in the wet end of the dryer. These partially top-felted single tier dryers are sold by Beloit Corporation under the trademark "BELRUN".
The series of top-felted dryer groups has not been continued all the way to the dry end of the dryer because each top-felted single tier dryer only directly contacts one side of the web--specifically, the bottom side--with the heated surfaces of the drying cylinders. A typical dryer including single tier sections has approximately 41 percent of the dryer cylinders in top-felted single tier groups, and the remaining 59 percent of the dryer cylinders are two tier, double felted dryer cylinders.
The actual and perceived problems associated with conventional two tier dryer groups and with dryers consisting entirely of top-felted single tier dryer groups have been addressed by the development of a dryer having alternating top-felted and bottom-felted single tier dryer groups, often running the full length of the dryer. Such dryers have been sold, for example, under the trademark "BEL-CHAMP" by Beloit Corporation. The alternating dryer groups, which usually are positioned in a generally horizontal, staggered series, alternately dry both sides of the web.
It was well known when alternating single tier dryers were developed that the sheet tends to curl towards the last side of the web to be dried, at least in laboratory studies. To be sure that both sides would dry at the same time, both sides were dried alternately, beginning in the very early stages of drying.
Concern about sheet curl led dryer group builders to dry alternate sides even in the very early stages in the alternating single tier dryers. For example, the #3 machine at CTS, Duino, Italy, was designed with the first three dryer cylinders top felted, the next three bottom felted, and the following three top felted. These first three groups were designed this short to ensure alternate-sided drying would be started in the very early stages of the drying process.
Those skilled in the art believed that preferentially drying the web on one side would create an imbalance in drying on the respective sides of the web, leading to a problem with sheet curl.
One difficulty with the alternating single tier dryer is that none of its dryers are stacked vertically by providing upper and lower tiers of dryers. Conventionally, the successive dryer cylinders are in a generally horizontal arrangement rather than in a two tier arrangement. Thus, the machine can be longer than earlier machines which have the same number and size of dryer cylinders.
The alternating single tier configuration also commonly requires an extra vacuum roll at each of the transfers from one dryer group to another.
In addition, although web breaks are infrequent in alternating single tier dryers, when the web does break, if it then wraps around a bottom felted dryer cylinder, the wrapped paper cannot be easily dumped into the basement. (The "basement" of a paper machine is the open space beneath the machine where the "broke" or useless, partially made paper produced by an unthreaded machine is collected for recycling in the machine.) Rather, the broke must be manually removed from the bottom felted dryer group by a worker. The manual removal of broke is time-consuming, and often must be done while the machine is stopped. Top-felted groups are open beneath the web, so broke automatically goes into the basement when the web breaks, or can be easily diverted there.
Also, in the alternating single tier arrangement, any one dryer group preferentially dries one side of the web. According to conventional thinking, the sheet must be reversed periodically by passing it through top-felted and bottom-felted groups to avoid curl. Conventional thinking has thus been that latent curl of the web cannot be controlled within the same group where it develops; it must be controlled by passing the web through a subsequent, oppositely felted group.